looks to me like this is a bigger issue than just the floor tile. Did the same "contractor" install the shower too? My biggest concern would be where the water is coming from.
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Tester101♦Jun 30 '11 at 16:56

When you say "picked up" do you mean the entire tile came off in one piece? A properly installed tile will pretty much always crack and be a real pain in the ass to remove and take lots of back-breaking scraping to get rid of the thinset from the subfloor. If you can (if you wanted to) rip all the tiles out in less than 20 minutes, it wasn't done right.
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gregmacSep 7 '12 at 21:50

2 Answers
2

You CAN bond tile directly to a plywood subfloor and expect it to last. HOWEVER, the floor must be properly reinforced to minimize flexing; the joists, if they're 2x8 (newer code) and not 2x10 (older code), must be braced every four feet with cross-braces, which minimizes independent flexing of each joist and the subfloor above. Then, in either case the subfloor should probably be about an inch thick (at least 3/4"), and if that's accomplished with more than one layer of plywood/OSB, the subfloor should be glued and screwed together such that they're as close to one piece of material as possible. If you have the vertical space for a floating underlayment like Ditra, that will further reduce grout and tile cracking by allowing the two dissimilar materials (wood and tile) to swell and shrink at their own differing rates with temperature and humidity.

I would not "glue" stone or ceramic tiles in any case; the tiles will likely not be properly supported under their entire area and WILL crack and lift. Use thinset, and since this is a bathroom I would use a waterproof membrane laid down over a first layer of thinset to protect the subfloor, as neither thinset nor grout are waterproof. A waterproof underlayment is flat-out required on the floor and up the walls in shower stalls and anywhere else you expect to have water sprayed or standing on a regular basis; the water will wick into the thinset by the gallon, and eventually contact the drywall, where it will eventually degrade and mold even water-resistant "green board".

so if I understand correct this is not so wrong that the floor needs to be pulled up and remade? or is it?
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kacalapyJun 29 '11 at 18:32

It shouldn't be so wrong that you have to tear the floor down to its joists; the bracing should be there for any upstairs floor, or else the house builders screwed you. However, you must beef up the subfloor to at least 3/4" total thickness OSB. That probably means adding a second 3/8" layer of OSB, gluing and screwing. I do not know if backer board is durable enough long-term to be a substitute for OSB of the proper thickness, but as it's supposed to do much the same job, I'd at least take a good look at using it vs subfloor as it will likely be firmer than OSB at the same thickness.
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KeithSJun 29 '11 at 18:44

ahh, well. the floor is finished and directly adhered to with thin set to the plywood. will this do or does the marble need to be taken up and then put on top of altered subfloor so it is thicker?
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kacalapyJun 29 '11 at 19:50

Without seeing it myself I can't say, but if you can jump up and down on that floor and not feel it give, I'd call it good and move on.
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KeithSJun 29 '11 at 20:26

Keith is right about the floor flex etc, however OSB is a poor choice for a subfloor. I prefer concrete board or "Hardy backer" for the final underlayment. It holds the bond with thinset well. Plywood or OSB needs to be isolated with waterproofing membrane in a bath as Keith mentioned.
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shirlock homesJun 30 '11 at 9:22

so if I understand correct this is not so wrong that the floor needs to be pulled up and remade? or is it?
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kacalapyJun 29 '11 at 18:32

If the tile is loose, then yes, you need to pull it all up and redo the floor.
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electricsauceJun 30 '11 at 3:07

Just to be clear, you don't have to pull up the plywood sub-floor and redo it. In this case, you should probably pull up the tile, clean up the mortar, install backer board properly, then re-tile.
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Chris VesperJun 30 '11 at 14:45